What you write in your booktrailer is completely up to you...it's what makes a trailer unique. But here are some tips to get started:
1. Use quotes out of the book. Did you read something that make you stop to read it again? Use it in a quote with an image to reflect it!
2. End with a rhetorical question. It'll make the viewer ask him/herself, "What happens next?" That's the response you're wanting. I usually put mine at the end of the trailer
3. For a more polished trailer, Do NOT add wording to every single slide! Your images should tell the story as much as your words. Putting words between every single slide takes away from a more elegant trailer.
4. Say it simply...and yes, use two word slides together if you need more room.